What is Weld Testing?

• Methods of weld testing and analysis are used

to assure the quality and correctness of the

weld after it is completed.

• This term generally refers to testing and

analysis focused on the quality and strength of

the weld

2

Why to test the weld?

• To ensure development of quality weld by

collecting qualitative and quantitative data.

– Qualitative - Non destructive tests

– Quantitative - Hardness, tensile strength, ductility,

toughness, fracture toughness

• To asses suitability of welding for specific

application.

3

How to test the weld?

• Stages of Inspection

– Before Welding

– During Welding

– After Welding

• Testing Techniques

– Destructive

– Non Destructive

4

Before Welding

1.Cleaning:

2. Edge Preparation

Baking of electrodes etc.

Surface Oxides,

grease, oils

removal

Dimensions

and Quality of

Edge

Removal of

Moisture from

electrode coating5

During Welding• Selection of input parameters like Current & Voltage, welding speed, shielding

gases, heat source temperature etc.

6

After Welding

• Removal of the slag

• Peening

– Stress removal

• Post Welding Treatment

– Refinement of grain structure & stress removal

Slag create problem in

multi - pass technique

7

Post Welding Heat Treatment

8

Above Recrystallization temperature for

refinement of grain structure in HAZ

Weld Testing

• Types

– Destructive

• Physical damage to w/p and welded join.

• Quantitative data obtained

– Non Destructive

• Without Physically damaging the workpiece and joint

• Qualitative data is obtained

9

Destructive Weld Testing

• Destructive test, some sort of the damage takes

place in the component which is being tested,

the extent of damage may be more or less, but

most of the time it is observed that component,

which has been tested by the destructive test is

damaged to such as extent that it cannot be

used for further, for the targeted application.

10

…Contd.

• These can be divided into two parts,

• Tests capable of being performed in the

Workshop.

• & Laboratory tests.

– microscopic-macroscopic , chemical and corrosive.

11

REASONS

Defects occur during welding which affect the

quality and hardness of the plate

Other defects occur through lack of knowledge

of and skill of the welder

For the training of welders

12

Types of Destructive Weld Testing

• Tensile Test

• Bend Test

• Hardness Test

• Toughness Testing

• Fatigue Behavior

13

Workshop

Laboratory

Tensile Testing

• Tensile test is used to check how the weld joint

will perform under tensile loading and under

different environment.

• Modulus of elasticity, Yield strength Ultimate

strength, kind of the deformation at the

different stages and the total elongation of the

weld joint, till the fracture.

• Most simple and common method.

14

Procedure

• Tensile Properties are

obtained in two ways

1. Taking specimen from

transverse direction of

weld joint consisting

base metal – heat

affected zone

ASTM A370 Mechanical testing of steel products.

ASTM E8 Tension testing of metallic materials.

15

16

• Tensile test results must be supported by stress

– strain diagram.

• Indicating modulus of elasticity, yield strength,

ultimate tensile strength etc.

17

Bad Good

2.All weld metal specimen

• This test is used to determine the tensileproperties of a specimen that consists entirely ofweld metal.

• The test specimen is oriented parallel to the weldaxis, and is machined entirely from the weldmetal.

• There are two reasons for performing an all weldmetal test:

-to qualify a filler metal or- determine the properties of the weld metal in

a particular weldment.

18

• The following are typically properties that are

measured and reported in an all weld metal

tension test.

- tensile strength

- yield strength

- elongation

19

Bend Test

• Bend test is one of the most important andcommonly used destructive test to determine,

– Ductility &,

– Soundness of the welded joints in terms ofporosity, inclusion, penetration & other macro sizeweld discontinuities.

• The outside of the bend is extensivelyplastically deformed so that any defects in, orembrittlement of, the material will be revealedby the premature failure of the coupon.

20

How to Bend?

• Bending of the weldjoint can be done fromface or root sidedepending upon thepurpose

• i.e. whether face or rootside is assessed.

• ASTM - E190-92 –Guided bend test forductility of welds

21

Types of Bend Test

• Free bend

– In free bend test between

the two supports, the

weld joint is placed and

then the compressive

load is applied for the

bending to take place.

– Cheaper

22

Guided bend test

• In the guided bendtest guided bendingis performed by,placing the weldjoint over the die.

• It offers, the bettercontrolledconditions of thespecimen and of theloading.

• Costlier than freebend test

23

Guided Bend

24

Loading in Bend Test

• Load keep on increasing until crack starts

appearing on face or root.

• Angle of bend is considered as measure of

ductility.

25

Good Bad

Hardness Test

• It is resistance to indentation or penetration.

Usually referred as a measure of resistance to

abrasion or scratch.

• Due to application of heat in welding,

materials like hard enable steels, castiorns are

subjected to hardening where as materials like

aluminum alloys (precipitation enabled)

become softer.

26

• Hardening and softening phenomenon usually

occurs at HAZ.

• The hardness test, is very simple test and it

gives the lot of information about, that if any

micro structural transformation has taken place

or any embrittlement has taken place due to the

application of the weld thermal cycle.

27

Toughness Test

• Toughness is the ability of a material to resist

both fracture and deformation.

• The toughness test simulates service

conditions often encountered by components

of the system used in transportation,

agricultural, and construction equipment.

• Can be determined by calculating energy

absorbed by material before fracture.

28

29

Methods of toughness testing

• Charpy Impact test

• Izod impact test

30

Charpy Impact Test

• The Charpy vee-notch impact test is the most

common fracture toughness test used by industry.

• A notched specimen is broken by a swinging

pendulum and the amount of energy required to

break the specimen is recorded in foot-pounds or

joules.

• This is determined by measuring how far the

pendulum swings upwards after it fractures the

specimen.

31

32

It can be seen that at low temperatures the

material is more brittle and impact toughness is

low. At high temperatures the material is more

ductile and impact toughness is higher.

The transition temperature is the boundary

between brittle and ductile behavior and this

temperature is often an extremely important

consideration in the selection of a material.

Izod Impact Test

• Testing is generally carried out with the

specimens at room temperature since the time

required to accurately place it in the machine

allows its temperature to increase.

• This can introduce a significant error when

conducting tests at various temperatures.

33

34

Fatigue Test

• ASTM E466 standard for preparing specimen.

• Fatigue performance of a component can be

determined by

– Endurance limit

– Number of load cycles that joint can withstand.

35

A fatigue test must include

• Type of Loading – Axial pulsating, reverse

bending or tension compression.

• Stress ratio – ratio of minimum stress to

maximum stress.

• Temperature and environment (ambient /

vacuum / corrosive)

• Type of sample.

36

1

2

Typical Dimensions of standard specimen

R = 100mm, Width (W) = 10.3mm, T =

11mm, Gripping Length = 50mm

• First step is to conduct tensile test for

assessing the yield strength of specimen.

• For plotting stress to number of cycles curve

(S-N Curve) fatigue test is conducted, with

maximum applied tensile load

corresponding to 0.9 times of yield strength

to determine number of load cycle required for

fracture.

• And in the same way the test is repeated for at

0.85, 0.8, 0.75, 0.7 etc.

37

S-N curve

38

Non Destructive Testing

• Nondestructive testing or Non-destructive

testing (NDT) is a wide group of analysis

techniques used in science and technology

industry to evaluate the properties of a

material, component or system without causing

damage.

39

What are Some Uses of NDE Methods?

• Flaw Detection and Evaluation

• Leak Detection

• Location Determination

• Dimensional Measurements

• Structure and Microstructure Characterization

• Estimation of Mechanical and Physical Properties

• Stress (Strain) and Dynamic Response Measurements

• Material Sorting and Chemical Composition Determination

Fluorescent penetrant indication

Why Nondestructive?• Test piece too precious to be destroyed

• Test piece to be reuse after inspection

• Test piece is in service

• For quality control purpose

Major types of NDT

• Detection of surface flaws

Visual

Magnetic Particle Inspection

Fluorescent Dye Penetrant Inspection

• Detection of internal flaws

Radiography

Ultrasonic Testing

Eddy current Testing

Most basic and common

inspection method.

Tools include

fiberscopes,

borescopes, magnifying

glasses and mirrors.

Robotic crawlers permit

observation in hazardous or

tight areas, such as air

ducts, reactors, pipelines.

Portable video inspection

unit with zoom allows

inspection of large tanks

and vessels, railroad tank

cars, sewer lines.

Visual Inspection

Dye Penetrant InspectionLiquid penetrant inspection (LPI) is one of the most

widely used nondestructive evaluation (NDE) methods.

Its popularity can be attributed to two main factors,

which are its relative ease of use and its flexibility. LPI

can be used to inspect almost any material provided that

its surface is not extremely rough or porous. Materials

that are commonly inspected using LPI include metals

(aluminum, copper, steel, titanium, etc.), glass, many

ceramic materials, rubber, and plastics.

• Liquid penetration inspection is a method that is used to reveal surfacebreaking flaws by bleedout of a colored or fluorescent dye from theflaw.

• The technique is based on the ability of a liquid to be drawn into a"clean" surface breaking flaw by capillary action.

• After a period of time called the "dwell," excess surface penetrant isremoved and a developer applied. This acts as a "blotter." It draws thepenetrant from the flaw to reveal its presence.

• Colored (contrast) penetrants require good white light while fluorescentpenetrants need to be used in darkened conditions with an ultraviolet"black light".

• Unlike MPI, this method can be used in non-ferromagnetic materials andeven non-metals

• Modern methods can reveal cracks 2m wide

• Standard: ASTM E165-80 Liquid Penetrant Inspection Method

Introduction

Why Liquid Penetrant Inspection?

• To improves the detectability of flaws

There are basically two ways that a

penetrant inspection process

makes flaws more easily seen.

(1) LPI produces a flaw indication

that is much larger and easier for

the eye to detect than the flaw

itself.

(2) LPI produces a flaw indication

with a high level of contrast

between the indication and the

background.

The advantage that a liquid

penetrant inspection (LPI) offers

over an unaided visual inspection is

that it makes defects easier to see

for the inspector.

Basic Steps

1.Cleaner2. Penetrant

+Dwell

3. Developer 4. Dwell

47

Surface preparations (Cleaner)

• One of the most critical steps of a liquid penetrantinspection is the surface preparation.

• The surface must be free of oil, grease, water, orother contaminants that may prevent penetrantfrom entering flaws.

• The sample may also require etching ifmechanical operations such as machining,sanding, or grit blasting have been performed.

• These and other mechanical operations can smearthe surface of the sample, thus closing the defects.

48

49

Cleaning

Applying Dye (penetrant)

• Once the surface has been thoroughly cleaned anddried, the penetrant material is applied byspraying, brushing, or immersing the parts in apenetrant bath.

• After application of penetrant the sample is leftpossible to be drawn from or to seep into a defectfor a sufficient time.

• The color of the penetrant material is of obviousimportance in a visible dye penetrant inspection,as the dye must provide good contrast against thedeveloper or part being inspected .

50

Penetrant TypesDye penetrants

– The liquids are coloured so thatthey provide good contrast againstthe developer

– Usually red liquid against whitedeveloper

– Observation performed in ordinarydaylight or good indoorillumination

Fluorescent penetrants

– Liquid contain additives to give

fluorescence under UV

– Object should be shielded from

visible light during inspection

– Fluorescent indications are easy to

see in the dark

Standard: Aerospace Material Specification (AMS)

2644.

Based on the strength ordetectability of the indication thatis produced for a number of verysmall and tight fatigue cracks,penetrants can be classified intofive sensitivity levels are shownbelow:

•Level ½ - Ultra Low Sensitivity

•Level 1 - Low Sensitivity

•Level 2 - Medium Sensitivity

•Level 3 - High Sensitivity

•Level 4 - Ultra-High Sensitivity

According to the method used toremove the excess penetrant fromthe part, the penetrants can beclassified into:

•Method A - Water Washable

•Method B - Post Emulsifiable, Lipophilic (get composed with solvent)

•Method C - Solvent Removable

•Method D - Post Emulsifiable, Hydrophilic (dissolve in water)

Further classification

Level goes on increasing , cost also

increases

Developer

The role of the developer is to pull the trapped penetrantmaterial out of defects and to spread the developer out on thesurface of the part so it can be seen by an inspector.

The fine developer particles both reflect and refract theincident ultraviolet light, allowing more of it to interact withthe penetrant, causing more efficient fluorescence.

The developer also allows more light to be emitted throughthe same mechanism. This is why indications are brighterthan the penetrant itself under UV light.

Another function that some developers performs is to create awhite background so there is a greater degree of contrastbetween the indication and the surrounding background.

• Dry powder developer –the least sensitive but

inexpensive

• Water soluble – consist of a group of chemicals that are

dissolved in water and form a developer layer when the

water is evaporated away.

• Water suspendible – consist of insoluble developer

particles suspended in water.

Nonaqueous – suspend the developer in a volatile

solvent and are typically applied with a spray gun.

Developer Types

Using dye and developer from different

manufacturers should be avoided.

Finding Leaks with Dye Penetrant

Advantages

• The method has high sensitive to small surface discontinuities.

• The method has few material limitations, i.e. metallic and

nonmetallic, magnetic and nonmagnetic, and conductive and

nonconductive materials may be inspected.

• Large areas and large volumes of parts/materials can be inspected

rapidly and at low cost.

• Parts with complex geometric shapes are routinely inspected.

• Indications are produced directly on the surface of the part and

constitute a visual representation of the flaw.

Aerosol spray cans make penetrant materials very portable.

• Penetrant materials and associated equipment are relatively

inexpensive.

Disadvantages

• Only surface breaking defects can be detected.

• Only materials with a relative nonporous surface can be inspected.

• Precleaning is critical as contaminants can mask defects.

• Metal smearing from machining, grinding, and grit or vapor blasting

must be removed prior to LPI.

• The inspector must have direct access to the surface being

inspected.

• Surface finish and roughness can affect inspection sensitivity.

• Multiple process operations must be performed and controlled.

• Post cleaning of acceptable parts or materials is required.

• Chemical handling and proper disposal is required.

Magnetic Particle Inspection (MPI)

• A nondestructive testing method used for defectdetection. Fast and relatively easy to apply and partsurface preparation is not as critical as for some otherNDT methods. – MPI one of the most widely utilizednondestructive testing methods.

• MPI uses magnetic fields and small magnetic particles,such as iron filings to detect flaws in components.

• The only requirement from an inspectability standpointis that the component being inspected must be made of aferromagnetic material such as iron, nickel, cobalt, orsome of their alloys. Ferromagnetic materials arematerials that can be magnetized to a level that willallow the inspection to be affective.

• The method is used to inspect a variety of

product forms such as castings, forgings, and

weldments.

• Many different industries use magnetic particle

inspection for determining a component's

fitness-for-use.

• Some examples of industries that use magnetic

particle inspection are the structural steel,

automotive, petrochemical, power generation,

and aerospace industries.

59

Basic PrinciplesIn theory, magnetic particle inspection (MPI) is a relatively

simple concept. It can be considered as a combination of

two nondestructive testing methods: magnetic flux leakage

testing and visual testing.

Consider a bar magnet. It has a magnetic field in and

around the magnet. Any place that a magnetic line of force

exits or enters the magnet is called a pole. A pole where a

magnetic line of force exits the magnet is called a north

pole and a pole where a line of force enters the magnet is

called a south pole.

Diamagnetic, Paramagnetic, and

Ferromagnetic MaterialsDiamagnetic metals: very weak and negative susceptibility

to magnetic fields. Diamagnetic materials are slightly

repelled by a magnetic field and the material does not retain

the magnetic properties when the external field is removed.

Paramagnetic metals: small and positive susceptibility to

magnetic fields. These materials are slightly attracted by a

magnetic field and the material does not retain the magnetic

properties when the external field is removed.

Ferromagnetic materials: large and positive susceptibility

to an external magnetic field. They exhibit a strong

attraction to magnetic fields and are able to retain their

magnetic properties after the external field has been

removed.

Ferromagnetic materials become magnetized when the magnetic

domains within the material are aligned. This can be done by

placing the material in a strong external magnetic field or by

passes electrical current through the material. Some or all of the

domains can become aligned. The more domains that are

aligned, the stronger the magnetic field in the material. When

all of the domains are aligned, the material is said to be

magnetically saturated. When a material is magnetically

saturated, no additional amount of external magnetization force

will cause an increase in its internal level of magnetization.

Unmagnetized material Magnetized material

Magnetizing the objectThere are a variety of methods that can be used to establish a

magnetic field in a component for evaluation using magnetic

particle inspection. It is common to classify the magnetizing

methods as either direct or indirect.

• Direct magnetization: current is passed directly through the

component.

Clamping the component between two electrical

contacts in a special piece of equipment Using clams or prods, which are attached or

placed in contact with the component

• Indirect magnetization: using a strong external magnetic field

to establish a magnetic field within the component

(a) permanent magnets

(b) Electromagnets

(c) coil shot

General Properties of Magnetic Lines of Force

• Follow the path of least resistance between

opposite magnetic poles.

• Never cross one another.

•All have the same strength.

• Their density decreases (they spread out)

when they move from an area of higher

permeability to an area of lower

permeability.

•Their density decreases with increasing

distance from the poles.

•Flow from the south pole to the north pole

within the material and north pole to south

pole in air.

When a bar magnet is broken in the center of its length, two complete bar

magnets with magnetic poles on each end of each piece will result. If the

magnet is just cracked but not broken completely in two, a north and south

pole will form at each edge of the crack.

The magnetic field exits the north

pole and reenters the at the south

pole. The magnetic field spreads out

when it encounter the small air gap

created by the crack because the air

can not support as much magnetic

field per unit volume as the magnet

can. When the field spreads out, it

appears to leak out of the material

and, thus, it is called a flux leakage

field.

If iron particles are sprinkled on a cracked magnet, the particles will

be attracted to and cluster not only at the poles at the ends of the

magnet but also at the poles at the edges of the crack. This cluster

of particles is much easier to see than the actual crack and this is

the basis for magnetic particle inspection.

Magnetic Particle Inspection

• The magnetic flux line close to the surface of a

ferromagnetic material tends to follow the surface

profile of the material

• Discontinuities (cracks or voids) of the material

perpendicular to the flux lines cause fringing of

the magnetic flux lines, i.e. flux leakage

• The leakage field can attract other ferromagnetic

particles

Cracks just below the

surface can also be

revealed

The magnetic particles

form a ridge many times

wider than the crack

itself, thus making the

otherwise invisible crack

visible

The effectiveness of MPI depends

strongly on the orientation of the

crack related to the flux lines

MPI is not sensitive to shallow and smooth

surface defects

Testing Procedure of MPI

• Cleaning

• Demagnetization

• Contrast dyes (e.g. white paint for dark particles)

• Magnetizing the object

• Addition of magnetic particles

• Illumination during inspection (e.g. UV lamp)

• Interpretation

• Demagnetization - prevent accumulation of iron

particles or influence to sensitive instruments

72

Some Standards for MPI Procedure

• British Standards

– BS M.35: Aerospace Series: Magnetic Particle Flaw Detection of

Materials and Components

– BS 4397: Methods for magnetic particle testing of welds

• ASTM Standards

– ASTM E 709-80: Standard Practice for Magnetic Particle

Examination

– ASTM E 125-63: Standard reference photographs for magnetic

particle indications on ferrous castings

• etc….

• One of the most dependable and sensitive methods for

surface defects

• fast, simple and inexpensive

• direct, visible indication on surface

• unaffected by possible deposits, e.g. oil, grease or other

metals chips, in the cracks

• can be used on painted objects

• surface preparation not required

• results readily documented with photo or tape impression

Advantages of MPI

Limitations of MPI

• Only good for ferromagnetic materials

• sub-surface defects will not always be indicated

• relative direction between the magnetic field and the

defect line is important

• objects must be demagnetized before and after the

examination

• the current magnetization may cause burn scars on the item

examined

Examples of visible dry magnetic particle indications

Indication of a crack in a saw blade Indication of cracks in a weldment

Before and after inspection pictures of

cracks emanating from a hole

Indication of cracks running between attachment holes in a hinge

Examples of Fluorescent Wet Magnetic

Particle Indications

Magnetic particle wet fluorescent

indication of a cracks in a drive shaft

Magnetic particle wet

fluorescent

indication of a crack

in a bearing

Magnetic particle wet fluorescent indication

of a cracks at a fastener hole

Radiography

Radiography involves the use of penetratinggamma- or X-radiation to examine material's andproduct's defects and internal features. An X-raymachine or radioactive isotope is used as a sourceof radiation. Radiation is directed through a partand onto film or other media. The resultingshadowgraph shows the internal features andsoundness of the part. Material thickness anddensity changes are indicated as lighter or darkerareas on the film. The darker areas in theradiograph below represent internal voids in thecomponent.

High Electrical Potential

Electrons

-+

X-ray Generator or

Radioactive Source

Creates Radiation

Exposure Recording Device

Radiation

Penetrate

the Sample

target X-rays

W

Vacuum

X-rays are part of the electromagnetic spectrum, with

wavelengths shorter than visible light.

X-rays are producedwhenever high-speedelectronscollide with a metaltarget.A source of electrons – hotW filament, a highaccelerating voltage(30-50kV) between the

cathode (W) and the anode

and a metal target.The anode is a water-cooledblock of Cu containingdesired target metal.

6o

81

Radiation sources

4.1.1 x-ray source

X-rays or gamma radiation is used

• X-rays are electromagnetic

radiation with very short

wavelength ( 10-8 -10-12 m)

• The energy of the x-ray can

be calculated with the

equation

E = h = hc/

h-plank constant c-speed of light - wavelength

e.g. the x-ray photon with

wavelength 1Å has energy

12.5 keV

Properties and Generation of X-ray

• All x-rays are absorbed to some extent in passing through matter

due to electron ejection or scattering.

• The absorption follows the equation

where I is the transmitted intensity;

x is the thickness of the matter;

is the linear absorption coefficient (element dependent);

is the density of the matter;

(/) is the mass absorption coefficient (cm2/gm).

Absorption of x-ray

xx eIeII

00

I0 I,

x

84

Radio Isotope (Gamma) Sources

Emitted gamma radiation is one of the three types of natural radioactivity. It

is the most energetic form of electromagnetic radiation, with a very short

wavelength of less than one-tenth of a nano-meter. Gamma rays are

essentially very energetic x-rays emitted by excited nuclei. They often

accompany alpha or beta particles, because a nucleus emitting those

particles may be left in an excited (higher-energy) state.

Man made sources are produced by introducing an extra neutron to atoms

of the source material. As the material rids itself of the neutron, energy is

released in the form of gamma rays. Two of the more common industrial

Gamma-ray sources are Iridium-192 and Colbalt-60. These isotopes emit

radiation in two or three discreet wavelengths. Cobalt 60 will emit a 1.33

and a 1.17 MeV gamma ray, and iridium-192 will emit 0.31, 0.47, and 0.60

MeV gamma rays.

Advantages of gamma ray sources include portability and the ability to

penetrate thick materials in a relativity short time.

Disadvantages include shielding requirements and safety considerations.

Film Radiography

Top view of developed film

X-ray film

The part is placed between the

radiation source and a piece of film.

The part will stop some of the

radiation. Thicker and more dense

area will stop more of the radiation.

= more exposure

= less exposure

• The film darkness (density) will

vary with the amount of radiation

reaching the film through the

test object.

• Defects, such as voids, cracks,

inclusions, etc., can be detected.

Contrast and Definition

It is essential that sufficient

contrast exist between the

defect of interest and the

surrounding area. There is no

viewing technique that can

extract information that does not

already exist in the original

radiograph

Contrast

The first subjective criteria for determining radiographic quality is

radiographic contrast. Essentially, radiographic contrast is the

degree of density difference between adjacent areas on a

radiograph.

low kilovoltage high kilovoltage

Definition or Quality of detail In image

Radiographic definition or quality is the abruptness of change in

going from one density to another.

Reliable poor

High definition: the detail portrayed in the radiograph is equivalent to

physical change present in the part. Hence, the imaging system must

produced a faithful visual reproduction.

Areas of Application

• Can be used in any situation when one wishes to view theinterior of an object

• To check for internal faults and construction defects, e.g.faulty welding

• To ‘see’ through what is inside an object

• To perform measurements of size, e.g. thickness measurementsof pipes

ASTM

–ASTM E94-84a Radiographic Testing

–ASTM E1032-85 Radiographic Examination of Weldments

–ASTM E1030-84 Radiographic Testing of Metallic Castings

Standard:

Radiographic Images

Limitations of Radiography

• There is an upper limit of thickness through which

the radiation can penetrate, e.g. -ray from Co-60 can

penetrate up to 150mm of steel

• The operator must have access to both sides of an

object

• Highly skilled operator is required because of the

potential health hazard of the energetic radiations

• Relative expensive equipment

Examples of radiographs

Cracking can be detected in a radiograph only the crack is

propagating in a direction that produced a change in thickness that

is parallel to the x-ray beam. Cracks will appear as jagged and

often very faint irregular lines. Cracks can sometimes appearing as

"tails" on inclusions or porosity.

Burn through (icicles) results when too much heat causes

excessive weld metal to penetrate the weld zone. Lumps of

metal sag through the weld creating a thick globular condition

on the back of the weld. On a radiograph, burn through

appears as dark spots surrounded by light globular areas.

Gas porosity or blow holes

are caused by accumulated

gas or air which is trapped by

the metal. These

discontinuities are usually

smooth-walled rounded

cavities of a spherical,

elongated or flattened shape.

Sand inclusions and dross

are nonmetallic oxides,

appearing on the radiograph

as irregular, dark blotches.

Ultrasonic Testing

The most commonly used

ultrasonic testing technique is

pulse echo, whereby sound is

introduced into a test object and

reflections (echoes) from internal

imperfections or the part's

geometrical surfaces are returned

to a receiver. The time interval

between the transmission and

reception of pulses give clues to

the internal structure of the

material.

In ultrasonic testing, high-frequency sound waves are transmitted into a

material to detect imperfections or to locate changes in material properties.

High frequency sound waves are introduced into a material and they arereflected back from surfaces or flaws.

Reflected sound energy is displayed versus time, and inspector canvisualize a cross section of the specimen showing the depth of features thatreflect sound.

f

plate

crack

0 2 4 6 8 10

initial

pulse

crack

echo

back surface

echo

Oscilloscope, or flaw

detector screen

Ultrasonic Inspection (Pulse-Echo)

Time

Fre

quen

cy

Generation of Ultrasonic Waves

• Piezoelectric transducers are used for

converting electrical pulses to mechanical

vibrations and vice versa

• Commonly used piezoelectric materials are

quartz, Li2SO4, and polarized ceramics such

as BaTiO3 and PbZrO3.

• Usually the transducers generate ultrasonic

waves with frequencies in the range 2.25 to

5.0 MHz

Equipment & TransducersPiezoelectric Transducers

The active element of most acoustic

transducers is piezoelectric ceramic.

This ceramic is the heart of the

transducer which converts electrical

to acoustic energy, and vice versa.

A thin wafer vibrates with a

wavelength that is twice its thickness,

therefore, piezoelectric crystals are

cut to a thickness that is 1/2 the

desired radiated wavelength.

Optimal impedance matching is

achieved by a matching layer with

thickness 1/4 wavelength.

Direction of wave

propagation

Impedance or Resistance

• Longitudinal waves

– Similar to audible sound

waves

– the only type of wave which

can travel through liquid

• Shear waves

– generated by passing the

ultrasonic beam through the

material at an angle

– Usually a plastic wedge is

used to couple the transducer

to the material

Characteristics of Piezoelectric Transducers

• Immersion: do not contact the

component. These transducers

are designed to operate in a

liquid environment and all

connections are watertight.

Wheel and squirter transducers

are examples of such immersion

applications.

Transducers are classified into groups according to the application.

Contact type

• Contact: are used for direct

contact inspections. Coupling

materials of water, grease, oils, or

commercial materials are used to

smooth rough surfaces and

prevent an air gap between the

transducer and the component

inspected.

immersion

• Dual Element: contain two independently

operating elements in a single housing.

One of the elements transmits and the

other receives. Dual element transducers

are very useful when making thickness

measurements of thin materials and when

inspecting for near surface defects.

Dual element• Angle Beam: and wedges are typically

used to introduce a refracted shear wave

into the test material. Transducers can be

purchased in a variety of fixed angles or in

adjustable versions where the user

determines the angles of incident and

refraction. They are used to generate

surface waves for use in detecting defects

on the surface of a component.

Angle beam

Ultrasonic Test Methods

• Fluid couplant or a fluid bath is needed for

effective transmission of ultrasonic from the

transducer to the material

• Straight beam contact search unit project a

beam of ultrasonic vibrations perpendicular to

the surface

• Angle beam contact units send ultrasonic

beam into the test material at a predetermined

angle to the surface

Normal Beam InspectionPulse-echo ultrasonic measurements can

determine the location of a discontinuity in

a part or structure by accurately

measuring the time required for a short

ultrasonic pulse generated by a

transducer to travel through a thickness of

material, reflect from the back or the

surface of a discontinuity, and be returned

to the transducer. In most applications,

this time interval is a few microseconds or

less.

𝒅 = 𝒗𝒕𝟐 𝒐𝒓 𝒗 = 𝟐𝒅

𝒕𝒅

where d is the distance from the surface

to the discontinuity in the test piece, v is

the velocity of sound waves in the

material, and t is the measured round-trip

transit time.

Angles beam inspection

• Can be used for testing

flat sheet and plate or pipe

and tubing

• Angle beam units are

designed to induce

vibrations in Lamb,

longitudinal, and shear

wave modes

Angle Beam Transducers and wedges are typically used to

introduce a refracted shear wave into the test material. An

angled sound path allows the sound beam to come in from

the side, thereby improving detectability of flaws in and

around welded areas.

The geometry of the sample below allows the sound

beam to be reflected from the back wall to improve

detectability of flaws in and around welded areas.

Crack Tip Diffraction

When the geometry of the part is relatively uncomplicated and the

orientation of a flaw is well known, the length (a) of a crack can be

determined by a technique known as tip diffraction. One common

application of the tip diffraction technique is to determine the length

of a crack originating from on the backside of a flat plate.

When an angle beam transducer

is scanned over the area of the

flaw, the principle echo comes

from the base of the crack to

locate the position of the flaw

(Image 1). A second, much

weaker echo comes from the tip

of the crack and since the

distance traveled by the

ultrasound is less, the second

signal appears earlier in time

on the scope (Image 2).

Crack height (a) is a function of the

ultrasound velocity (v) in the

material, the incident angle (2)

and the difference in arrival times

between the two signal (dt).

The variable dt is really the

difference in time but can easily be

converted to a distance by dividing

the time in half (to get the one-way

travel time) and multiplying this

value by the velocity of the sound

in the material. Using trigonometry

an equation for estimating crack

height from these variables can be

derived.

Surface Wave Contact Units

• With increasedincident angle so thatthe refracted angle is90°

• Surface waves areinfluenced most bydefects close to thesurface

• Will travel alonggradual curves withlittle or no reflectionfrom the curve

Data Presentation

Ultrasonic data can be collected and displayed

in a number of different formats. The three most

common formats are know in the NDT world as

A-scan, B-scan and C-scan presentations.

Each presentation mode provides a different

way of looking at and evaluating the region of

material being inspected. Modern computerized

ultrasonic scanning systems can display data in

all three presentation forms simultaneously

A-Scan

The A-scan presentation displays the amount of received

ultrasonic energy as a function of time. The relative amount of

received energy is plotted along the vertical axis and elapsed

time (which may be related to the sound energy travel time

within the material) is display along the horizontal axis.

Relative discontinuity

size can be estimated by

comparing the signal

amplitude obtained from

an unknown reflector to

that from a known reflector.

Reflector depth can be

determined by the position

of the signal on the

horizontal sweep.

The B-scan presentations is a profile (cross-sectional) view of the a test

specimen. In the B-scan, the time-of-flight (travel time) of the sound energy

is displayed along the vertical and the linear position of the transducer is

displayed along the horizontal axis. From the B-scan, the depth of the

reflector and its approximate linear dimensions in the scan direction can be

determined.

B-Scan

C-ScanThe C-scan presentation provides a plan-type view of the location

and size of test specimen features. The plane of the image is parallel

to the scan pattern of the transducer.

The relative signal amplitude or

the time-of-flight is displayed as a

shade of gray or a color for each

of the positions where data was

recorded. The C-scan presentation

provides an image of the features

that reflect and scatter the sound

within and on the surfaces of the

test piece.

Gray scale image produced using

the sound reflected from the front

surface of the coin

Gray scale image produced using the

sound reflected from the back surface

of the coin (inspected from “heads” side)

High resolution scan can produce very detailed images.

Both images were produced using a pulse-echo

techniques with the transducer scanned over the head

side in an immersion scanning system.

• Eddy current testing can be used on all electrically conducting materials

with a reasonably smooth surface.

• The test equipment consists of a generator (AC power supply), a test coil

and recording equipment, e.g. a galvanometer or an oscilloscope

• Used for crack detection, material thickness measurement (corrosion

detection), sorting materials, coating thickness measurement, metal

detection, etc.

Eddy Current TestingElectrical currents are generated in a conductive material by an

induced alternating magnetic field. The electrical currents are

called eddy currents because the flow in circles at and just

below the surface of the material. Interruptions in the flow of

eddy currents, caused by imperfections, dimensional changes,

or changes in the material's conductive and permeability

properties, can be detected with the proper equipment.

Principle of Eddy Current Testing (I)

• When a AC passes through a test

coil, a primary magnetic field is

set up around the coil

• The AC primary field induces

eddy current in the test object

held below the test coil

• A secondary magnetic field arises

due to the eddy current

Mutual Inductance

(The Basis for Eddy Current Inspection)

The flux B through circuits as the sum of two parts.

B1 = L1i1 + i2M

B2 = L2i2 + i1M

L1 and L2 represent the self inductance of each of the coils. The constant

M, called the mutual inductance of the two circuits and it is dependent on

the geometrical arrangement of both circuits.

The magnetic field produced by circuit 1

will intersect the wire in circuit 2 and

create current flow. The induced current

flow in circuit 2 will have its own

magnetic field which will interact with

the magnetic field of circuit 1. At some

point P on the magnetic field consists of

a part due to i1 and a part due to i2. These

fields are proportional to the currents

producing them.

• The strength of the secondaryfield depends on electrical andmagnetic properties, structuralintegrity, etc., of the test object

• If cracks or otherinhomogeneities are present,the eddy current, and hence thesecondary field is affected.

Principle of Eddy Current Testing (II)

• The changes in the secondary

field will be a ‘feedback’ to the

primary coil and affect the

primary current.

• The variations of the primary

current can be easily detected

by a simple circuit which is

zeroed properly beforehand

Principle of Eddy Current Testing (III)

Conductivematerial

CoilCoil's magnetic field

Eddy currents

Eddy current's magnetic field

Eddy Current Instruments

Voltmeter

Eddy currents are closed loops of induced current circulating in planes

perpendicular to the magnetic flux. They normally travel parallel to the

coil's winding and flow is limited to the area of the inducing magnetic field.

Eddy currents concentrate near the surface adjacent to an excitation coil

and their strength decreases with distance from the coil as shown in the

image. Eddy current density decreases exponentially with depth. This

phenomenon is known as the skin effect.

Depth of Penetration

The depth at which eddy current density has decreased to 1/e, or about 37%

of the surface density, is called the standard depth of penetration ().

Three Major Types of Probes

• The test coils are commonly

used in three configurations

– Surface probe

– Internal bobbin probe

– Encircling probe

Result presentation

The impedance plane

diagram is a very useful

way of displaying eddy

current data. The strength

of the eddy currents and

the magnetic permeability

of the test material cause

the eddy current signal on

the impedance plane to

react in a variety of

different ways.

•Crack Detection

•Material Thickness

Measurements

•Coating Thickness

Measurements

•Conductivity Measurements For:

•Material Identification

•Heat Damage Detection

•Case Depth Determination

•Heat Treatment Monitoring

Applications

Surface Breaking CracksEddy current inspection is an excellent

method for detecting surface and near

surface defects when the probable defect

location and orientation is well known.

In the lower image, there is a

flaw under the right side of

the coil and it can be see that

the eddy currents are weaker

in this area.

Successful detection requires:

1. A knowledge of probable defect type, position, and

orientation.

2. Selection of the proper probe. The probe should fit the

geometry of the part and the coil must produce eddy

currents that will be disrupted by the flaw.

3. Selection of a reasonable probe drive frequency. For

surface flaws, the frequency should be as high as

possible for maximum resolution and high sensitivity.

For subsurface flaws, lower frequencies are necessary

to get the required depth of penetration.

Applications with Encircling

Probes

• Mainly for automatic productioncontrol

• Round bars, pipes, wires andsimilar items are generallyinspected with encircling probes

• Discontinuities and dimensionalchanges can be revealed

• In-situ monitoring of wires usedon cranes, elevators, towingcables is also an usefulapplication

Applications with Internal Bobbin

Probes

• Primarily for

examination of tubes in

heat exchangers and oil

pipes

• Become increasingly

popular due to the wide

acceptance of the

philosophy of

preventive maintenance

Applications with Internal Bobbin

Probes

•Sensitive to small cracks and other defects

•Detects surface and near surface defects

•Inspection gives immediate results

•Equipment is very portable

•Method can be used for much more than flaw detection

•Minimum part preparation is required

•Test probe does not need to contact the part

•Inspects complex shapes and sizes of conductive

materials

Advantages of ET

•Only conductive materials can be inspected

•Surface must be accessible to the probe

•Skill and training required is more extensive than other

techniques

•Surface finish and and roughness may interfere

•Reference standards needed for setup

•Depth of penetration is limited

•Flaws such as delaminations that lie parallel to the

probe coil winding and probe scan direction are

undetectable

Limitations of ET